Field of the Invention
This invention relates generally to an optical communications system and more particularly to an optical communications system for modulating in real-time the wavefront of a beam from a coherent optical source.
Background Description
In optical communications, there are applications where the wavefront of a beam may change as a function of time. Laser diodes and laser diode arrays (LDA's), which are uniquely suited to satellite communications due to their low mass and compactness, are known to degrade with age, suffering changes in frequency and beamfront characteristics. Propagation through the atmosphere induces changes in the wavefront due to changes in the index of refraction as a function of time. In these cases and others, the optical solutions employed to correct and direct a given beam would suffer degradation in effectiveness over time. Active or adaptive optical systems employ real-time control over optical wavefronts to optimize overall system performance in the presence of time-dependent changes in the beamfront. For applications where maintaining wavefront fidelity throughout a given optical link is required to retain high signal-to-noise ratios, beamfront fidelity or good image reconstruction capability, incorporation of such techniques into the overall system design is highly desirable. Specifically, for the application to spacecraft laser communications, real-time, or dynamic, holography can provide a low mass, compact solution with potentially high resolution for real-time beamfront correction.
Wavefront Aberration Function: In detecting and analyzing changes in wavefronts, the commonly used figure of merit is the Strehl ratio. The Strehl ratio describes the ratio of peak intensity of an image to that of an unaberrated wavefront, and is defined as: ##EQU1## where I(P) refers to intensity at point P defined as: ##EQU2## I is the intensity of the unaberrated system which is assumed to be a perfect wavefront, i.e. a sphere of constant amplitude: ##EQU3## where A is the amplitude and R is the radius of the reference sphere, .lambda. is the wavelength and .phi. is the aberration function with respect to the spherical wavefronts. FIG. 1A illustrates the model of the imaging system. For small aberrations, the wavefront quality can be specified in terms of variance from this reference spherical wavefront. The wavefront aberration function at point Q is the difference in the aberrated wavefront with respect to the ideal spherical reference circle, S. The resulting disturbance at image point, P, is obtained by integrating all elements of d.phi. over ds as illustrated in FIG. 1B. It can be demonstrated that small rms errors will result in significant degradation in peak intensity from the ideal as disclosed in Prin. of Optics, 2nd Ed., Pergamon Press, 1965 (460-462) by M. Born and E. Wolf. Hence, the signal-to-noise ratios and more importantly, the bit error rate (BER), which is critical to an optical communications link, is strongly affected. Large aberrations result in degradation of the image structure. Hence, recovery of beam fidelity both to retain high signal-to-noise ratios, low bit error rates, and wavefront quality motivates the development of effective methods to compensate in real-time distortions which degrade overall system performance.
System Considerations: FIG. 2 is a block diagram illustrating the basic components of a system to detect and compensate for beamfront distortion which may be induced from the transmitter and/or channel effects. The fundamental elements of an active or adaptive optical system which can correct the wavefront of the beam must include a means to detect and measure the changes in the beam wavefront 2, a means to analyze and specify a compensatory wavefront, and hardware 6 to modify the distorted wavefront in a specified way. These elements are common to both active optics, which has come to be used to describe open-loop control of wavefront correction, and adaptive optics, which has come to refer to more complex wavefront correction which usually operates in a closed-loop mode. A system detector 10 receives a corrected wavefront beam.
Laser diodes and diode arrays are uniquely suited to spacecraft communications due to their low mass and compact size. These types of optical transmitters, however, suffer beamfront aberrations characterized typically by astigmatism and ellipticity as well as wide divergence. A typical laser output is shown in FIG. 3. Configurations which use conventional optical components to correct and direct the beam are massive and bulky and provide approximate solutions for a specific communications beam. Holograms, due to their unique characteristic of incorporating the functions of multiple optical elements into a single interference pattern which can reconstruct both recorded amplitude and phase information, offer a compelling alternative as disclosed in an article in Applied Optics. 24, 2150(1985) by W. H. Carter and H. J. Caulfield. Ideally, one would illuminate a hologram made to correct and direct the output from a specific diode transmitter with the uncorrected beam and the corrected "communications" beam would be diffracted from the hologram at the Bragg angle. FIG. 4A compares the conventional optical solution with the holographic approach in FIG. 4B. In FIG. 4A, a laser 20 transmits an optical signal which passes through a collimating lens 22, through an anamorphic prism pair 24, and through correcting optics 26 to give the desired wavefront before reaching receiving optics 28. In FIG. 4B, the laser 20 transmits an optical signal 20 which passes through a partially collimating lens 22, and through a hologram 30 to give the desired resultant wavefront beam.
As previously discussed, changes in output beam propagation characteristics due either to transmitter degradation over time or distortions induced by disturbances in the optical path motivate a solution wherein modulation of the wavefront to compensate for these distortions is accomplished "in situ" in real-time.
In spacecraft communications, high resolution, device durability, erasability, low power consumption, compactness, relatively low mass and maximized diffraction efficiency are critical design criteria which are influenced by both the functional setting (i.e. the unique requirements of satellite-to-satellite communications) and by the characteristics of the optical transmitter selected.
The most straightforward approach to beam wavefront correction consists of a technique to directly modulate a distorted wavefront signal beam in a given medium by converting the digital signals resulting from analysis into a specific modulation using a two-dimensional spatial light modulator (2-D SLM). The modulated material must be able to respond to light radiating in the near-infrared with high efficiency. FIG. 5 illustrates this strategy. As shown, the coherent optical transmitter 40, characterized as a diode or diode array, illuminates a 2-D SLM 42 which can be modified in place in real-time by electrical distributions derived from a computer 46 analysis of the beamfront aberrations through an interface 44.
Detection of wavefront aberrations and feedback strategies are not addressed here. However, a number of designs ar under investigation, e.g. J. W. Hardy, in IEEE Proc., 66, 651, (1978, presents an excellent introductory review of techniques. Also, the interference phase loop (IPL) developed by Fisher and Warde in opt. Lett., 4, 131 (1979), is a comparatively direct approach which can be adapted to a number of system configurations which employ SLM's.
Assuming variations in a given phasefront have been detected and the information transmitted to a wavefront analyzer, wavefront analysis must be conducted to characterize the distortion and then predict the compensatory grating which must be induced in hardware. In order to reduce payload, should an onboard processing design be elected, and/or to meet response time constraints, it is clear that a reduced number of computational steps which can be accomplished through advanced signal and parallel processing techniques is desirable. The methodology to be employed, however, will be, in part, determined by the type of hardware implementation selected.
Techniques to directly induce phase modulation such as on a monolithic device using the electro-optic (E-O) or magneto-optic effect, are attractive technologies which enable translation of wavefront analysis into hardware implementation. However the lack of commercially available SLM's which can provide high resolution, dynamic range, fast response time, high diffraction efficiency and are robust, modular, and have low power consumption prevent implementation of the direct approach. It is desirable, then, to relax the operational parameters which would make available a wider class of SLM strategies and devices.